Because of being too hard and thereby exhibiting almost no workability, a steel strip after cold rolling cannot be subjected to press forming and hence cannot be put into practical use so long as no further treatment is effected. To improve the workability of the steel strip, it is necessary to increase the grain size of the steel strip sufficiently, and to reduce the amount of solid-solution state carbon contained in the steel strip as small as possible.
To this end, continuous annealing treatment including soaking, primary cooling and averaging is performed. More specifically, a steel strip after cold rolling is heated above the recrystallization temperature and is kept at the soaking temperature of 700.degree.-850.degree. C. for a certain period of time for growth of crystal grains. During the soaking step, the dissolved carbon is generated in solid-solution state, and it must be settled to be harmless in the succeeding steps. Therefore, in the first half of the primary cooling, the steel strip is slowly cooled down to a certain temperature (600.degree.-700.degree. C.) so as to increase the amount of solid-solution state carbon in the ferrite matrix, and to prevent deterioration of flatness of the steel strip such as cooling buckle for achieving satisfactory operation. In the second half of the primary cooling, the steel strip is rapidly cooled down to the overaging temperature (about 400.degree. C.). Then the steel strip is kept at the overaging temperature for a certain period of time so that the solid-solution state carbon is precipitated as cementite for reducing the amount thereof. Lastly the steel strip is subjected to the final cooling.
Where cooling by water-gas mixture is performed in the second half of the primary cooling to rapidly cool the steel strip, an oxide film is formed on the surface of the steel strip so that a post-treatment such as acid pickling is required immediately after the continuous annealing. In view of the above, a method of cooling a steel strip by blowing inert atmosphere gas on N.sub.2 basis upon it wherein the cooling gas contains a large amount of H.sub.2 gas that has a great cooling ability, is proposed in Japanese Patent Publication No. Sho 55-1969 and Japanese Patent Laid-Open Publication No. Hei 6-346156, for example.
Although the above-cited Publications suggest the concentration of H.sub.2 gas being adjusted within the range of 8-90%, the following problems have not yet been resolved satisfactorily.
(1) While an increase in the concentration of H.sub.2 gas surely increases the cooling ability, such an increase of the H.sub.2 gas concentration pushes up the running cost.
(2) If the concentration of H.sub.2 gas is lowered and the blowoff speed of the gas from nozzles is raised, a higher cooling ability can be achieved. However, if the blowoff speed exceeds a certain value, the running steel strip tends to flutter and suffer from scratches on the surface thereof.
(3) Also, if the cooling gas containing H.sub.2 gas is recirculated and only the loss is supplied, the cooling gas can be used efficiently. In this case, however, since the cooling gas is heated by the steel strip to be cooled and consequently becomes a high temperature gas, the cooling efficiency of cooling gas is lowered. In this manner, the appropriate temperature of the cooling gas has not been taken into consideration in the prior art.
The present invention has been made in view of these drawbacks of the conventional method, and its object is to provide a primary cooling method in continuously annealing steel strip more efficiently and in a more inexpensive manner, wherein the concentration of H.sub.2 gas, the temperature of cooling gas and the blowing speed of the cooling gas are appropriately selected on the basis of the results of various experiments.